1. Field of the Invention
The present invention relates generally to photoconductive switches and more particularly to photoconductive bulk gallium arsenide switches for use in impulse radars, active counter measure apparatus and high power microwave generation devices.
2. Description of the Prior Art
It is generally known that a relatively high dark resistivity semiconductor can be transformed from a semi-insulating state to a quasi-metallic state almost instantaneously when it is illuminated with optical pulses of a proper wavelength. Due to this property, high resistivity semiconductors have been widely used as photoconductive optically activated switches because they are able to provide fast risetime, low jitter, and high power capabilities. These qualities have proved most advantageous in devices that produce high intensity microwaves.
A typical prior art photoconductive switch of this type is shown and described, for example, in U.S. Pat. No. 5,028,971, entitled, "High Power Photoconductor Bulk GaAs Switch", which issued to A.H. Kim et al on Jul. 2, 1991. The teachings of this patent are meant to be specifically incorporated herein by reference and comprise a bulk piece of gallium arsenide which is masked with a grided electrode on either one or both sides of the semiconductor body. The grided electrodes are electrically connected to a power supply source and an electric field is distributed substantially uniformly over the entire active switching area. The pattern of electrodes is designed to permit as much light penetration as possible through the active area of the semiconductor. When laser light is introduced to the switch parallel to the applied electric field and having an effective gap in the order of 5 mm-7 mm, the semiconductor changes from an off state to an on state in less than a nanosecond. The optical illumination for activating the switch typically comprises pulses of a Q switched neodymium doped yttrium aluminum garnet (Yd:YAG) laser generating an output of 0.8 mj at a wavelength of 1.06 microns.
In the above-referenced related application which is also meant to be incorporated herein by reference, there is disclosed an optically activated sub-nanosecond hybrid pulser using radial and quasi-radial transmission lines and which incorporate a photoconductive GaAs switch device as described above. Depending on the pulsewidth requirement and the structure of the radial transmission line, a voltage gain can be obtained without degradation of the pulse risetime. Where a solid state laser diode is used as the optical light source, its relatively low optical output energy and short wavelength output requires an extremely efficient optically activated switch.